The National Institutes of Health (NIH) and National Institute of Environmental Health and Science (NIEHS) have recently published a White Paper emphasizing the need to study the impact of climate change on human health, specifically among vulnerable populations including pregnant women. Pre-term delivery (PTD) is the leading cause of infant mortality and morbidity and among the top public health challenges facing our nation as well as the rest of the world. Yet the etiology remains largely unknown. Studies of air pollution and seasonal trends have supported a relationship between adverse birth outcomes, such as PTD, thus supporting the need for more research to elucidate this relationship. Pregnant women may be at increased risk of PTD during periods of heat exposure, since they are more likely to be dehydrated, which could result in decreased uterine blood flow and increased pituitary secretion of anti-diuretic hormone and oxytocin to induce labor. The proposed study examines an innovative hypothesis and a previously unknown risk factor for PTD that has newly emerged from the literature. Specifically, the objective of the proposed study is to examine the impact of climate change on the risk of PTD, and will be among the first such studies designed to address this association in a clinical setting. To achieve the proposed aims, we will conduct a case-crossover study utilizing members of Kaiser Permanente Northern California (KPNC), an integrated health care delivery system with more than 33,000 deliveries each year. The case-crossover is a study design uniquely suited for examining the association between short-term high ambient temperature and the risk of PTD, as it has the advantage of eliminating potential measured and unmeasured confounders inherently by study design. The proposed study will use data on deliveries that occurred between January 1995 and December 2009 from KPNCs electronic medical records (EMR), which includes information on residence at the time of deliveries. Data on maternal residential apparent temperature, a measure of both temperature and relative humidity combined, will be provided by the California Irrigation and Management System and the US Environmental Protection Agency. A cohort of approximately half a million births will be identified through KPNC's EMR, and approximately ten percent of these births are expected to be preterm. The analyses will be stratified by season, and also by maternal race/ethnic group, maternal age, and socioeconomic status. Findings from this study will contribute to the nascent understanding of this relationship. Once the relationship between increased risk of PTD and apparent temperature has been established, public health interventions can be implemented to reduce the impact of increasing heat exposure on pregnant women, while targeting those who are at greatest risk. Precautions may include drinking more fluids and electrolytes, staying indoors in air conditioned environments, and being with others during the warm season, especially during heat waves.